The present invention relates generally to high-power laser systems, an in particular to systems that combine the beams of multiple lasers to expand output power and bandwidth.
High-power, high-brightness lasers are utilized in a wide variety of industrial, optical, and medical applications. For many such applications laser power is carried by optical fibers, which can be single-mode or multimode in nature, depending on the application. The light-carrying cores of single-mode fibers are narrower than those of multimode fibers. Because light travels more quickly through smaller cores and also suffers less attenuation, single-mode fibers are preferred for applications involving high-frequency light pulses and long travel distances.
In order to couple to a small fiber core (the size of which is typically expressed as an xe2x80x9cxc3xa9tendue,xe2x80x9d i.e., the product of the fiber""s core diameter and numerical aperture), a laser must have a small-diameter, narrow-divergence output beam and, therefore, a high brightness level. Unfortunately, typical high-power lasers do not exhibit sufficient brightness levels to permit coupling into single-mode fibers.
Copending U.S. Ser. No. 09/149,610 (filed Sep. 8, 1998) now U.S. Pat. No. 6,208,679 U.S. Ser. No. 09/337,081 (filed Jun. 21, 1999), and U.S. Ser. No. 09/498,462 (filed Feb. 4, 2000) now U.S. Pat. No. 6,192,062 describe external-cavity laser designs that generate coaxially overlapping outputs at multiple wavelengths. For example, an external laser resonator may be based on a bar of light-emitting semiconductor material whose outputs emerge from a linear sequence of stripes along the length of the bar. These outputs pass through an output-coupling lens and strike a dispersive element, such as a diffraction grating. Light dispersed by the dispersive element is reflected by a partial mirror back along the optical path, passing through the lens and returning to the semiconductor outputs, the opposite facets of which are reflective. The resulting feedback produces laser amplification, and light not reflected by the partial mirror represents the output of the system.
The reflective semiconductor facets and the partial mirror together form an ensemble of individual, external-cavity lasers, each with its own optical path. The lens and dispersive element force the individual beams into a coaxial configuration, their paths intercepting at the dispersive element. Moreover, because the beam of each of these lasers strikes the dispersive element at a different angle, each laser has a different optical path and, therefore, resonates at a different wavelength. The overall result is a high-power, multi-wavelength beam with high brightness due to the coaxially overlapping component beams.
Although this configuration produces high output power levels, those levels ultimately depend on the inputs to the systemxe2x80x94i.e., the light produced by the semiconductor bar. And because the power of semiconductor emitters is limited, overall system power will be limited as well.
In accordance with the present invention, either or both of two strategies is employed to increase system output. In one approach, the external-resonator design is modified to allow each input to undergo individual amplification notwithstanding optical beam combination. In this way, overall output power may be scaled in a desired fashion, depending on the selected characteristics of the optical amplifier elements.
To achieve additional power, each of the amplifiers may implemented as a phased array. Viewed more generally, this configuration affords beam combining in two stages, with each contributing input source itself composed of multiple sources whose outputs have been combined. If each phased-array source emits at a different wavelength (i.e., has a different output wavelength profile), this design offers a multi-wavelength output whose power level may be scaled in accordance with the number and character of the devices forming each phased array.
Thus, in one aspect, the invention comprises a multi-wavelength light-generation system based on an optical source that produces a plurality of spatially separated optical outputs having different wavelength profiles. An optical amplifier having a plurality of optical gain elements receives the optical outputs and amplifies them. The amplified outputs are directed onto a dispersive optical device, which spatially combines them in the near and far fields, resulting in a co-propagated multi-spectral output.
In another aspect, the invention comprises a multi-wavelength light-generation system based on a series of phased-array optical gain sources. In particular, each of the gain sources utilizes the combined outputs of a plurality of constituent radiation sources arranged in a phased array. Each of the various phased-array gain sources produces an output having a different wavelength profile. These outputs are spatially combined by a dispersive optical device.